We report on the results of the investigation of optical properties and structure of PECVD deposited thin films of hydrogenated polysilicon determined by UV-Vis and IR spectroscopy. The influence of the hydrogen dilution of silane plasma at the PEVCD deposition on the film properties was investigated. The refractive index, the optical band gap energy and the microstructure of hydrogen and oxygen were analysed. The changes are discussed and correlated with the structure, the changes of the surface morphology and the hydrogen to silicon bonding. The optical band gap becomes larger than that of the undiluted sample. The results show that at dilution between 20 and 30 the transition between amorphous and crystalline phase occurs and the sample becomes a mixture of amorphous, polycrystalline phase with nano-sized grains and voids with decreasing hydrogen concentration. The presence of interstitial oxygen and oxygen bonded in surface SiOH groups was detected. (c) 2005 Elsevier Ltd. All rights reserved.